|Parents||Sean O'Reilly (father)|
Aurelia Davidson (mother)
|Siblings||Julius O'Reilly (half-brother)|
Trajan O'Reilly (half-brother)
He was born in 3049. Mother and son were subsequently installed in a sumptuously furnished wing of the Imperial Palace on Alphard, alongside at least two other mothers of the Imperator's other children.
Only Marcus's first name is known. Since he was an illegitimate son born to one of Imperator O'Reilly's mistresses, it is uncertain if he really bore the O'Reilly surname.
- The Periphery, Second Edition, p. 56, "Sean's Accession"